The invention disclosed herein is an improvement over U.S. Pat. No. 4,509,426 which issued on Apr. 9, 1985 to the inventor of the present application. The entire disclosure of this patent is incorporated herein by reference.
The invention is a roller that is adapted for rotating and oscillating axially while being in contact with other rollers in a machine such as in the inker of an offset printing press or in machines for applying a thin coating of material to metal and non-metallic sheets and webs. For the sake of brevity the improved axially oscillating roller will be described herein in connection with its printing press application where it applies or coats ink on sheets but it should be understood that the ink could be replaced by paint, varnish or polymer coating material in other applications.
The objective of using one or more oscillating ink rollers in the inker of offset printing presses is to obtain a uniform distribution of ink on the rollers that apply ink to the photolithographic image plates. In a printing press, at least one of the ink rollers is in tangential contact over its length with another roller that rotates in an ink bath and gets a heavy coating of ink on it. The next roller pressing against the roller in the ink bath, called the ductor roller, picks up a thinner coating of ink and transfers it to the series of rollers which, in turn, transfer and spread the ink from one roller to another until at the end of a series of rollers a thin and hopefully uniform coating exists on the last stage of rollers which are the form rollers. The form rollers make tangential contact with the image plates on the plate cylinder and, thus, deposit an ink film of one color on the image plate. As the plate cylinder rotates, it transfers the impression to a blanket cylinder for further transfer of printing on the paper sheet or web as is well known. An impression cylinder presses the paper against the blanket cylinder. The axially oscillating ink roller described in the cited patent became highly regarded by the printing industry very quickly because of its capability for obtaining not heretofore achieved uniform ink film thickness on the form rollers. The users, thus, enjoyed elimination of artifacts and, particularly, ghosting in printed sheets and webs.
The basic self-driven axially oscillating roller embodiments described in the cited patent operate on a shaft which spans across the width of a printing press. The ink roller is journaled for rotation and for reciprocating axially within limits on the shaft. The shaft is surrounded by an ink roller. A sleeve of plastic or other strong lighter than steel material fits concentrically inside of the roller. One end of the sleeve has an internal right hand square thread or helical groove in it and the axially adjacent part of the sleeve has a square thread in it. A rocker arm extends axially along the shaft and it is mounted to the shaft for pivoting about its mid-point. There is a prong at each end of the lever. When the lever rocks so that one of the prongs is engaged with the helix of one twist and the other prong is disengaged from its cooperating helix, the roller will be shifted axially in one direction because it is being rotated about its axis by a tangentially contacting adjacent roller which is power driven or receives its rotational force from other rollers in the inker. There are strikers in the sleeve at the ends of the left and right hand helixes. When the roller screws onto a prong sufficiently for the prong to reach a striker, the prong is driven out of engagement with the helix by that striker and the other prong is rocked into the oppositely twisted helix. The roller then shifts in the opposite axial direction as it continues to rotate.
Rollers of the type described in the cited patent have proved that they are capable of reducing ghosts and other artifacts by obtaining more uniform distribution of ink on the image plates. The rollers under discussion were found to have satisfactory life when used in presses that pass sheets for printing through at about 600 lineal feet per minute. At 600 lineal or peripheral feet per minute, a roller having a four inch diameter will have a rotational speed of about 573 rpm. A roller embodiment such as one of the two described in the cited patent can be designed with sufficiently low mass so that the almost instantaneous axial reversals do not set up noticeable shock or vibrations and do not put so much strain on the parts that they break or wear out prematurely. In the patented designs that were used commercially, the pitch of the square threads had to be a minimum of 1/4 inch in order to provide the strength needed to withstand the shock due to the reversals. The unavoidably large thread pitch is acceptable in oscillating rollers of about three or more inches in diameter because the amount of axial movement per revolution of the roller is small and the number of revolutions between reversals is relatively small. However, in small diameter rollers axial travel per revolution is high when the helix or thread pitch is large which means that for any selected amount of axial travel the number of reversals will be greater. It is desirable to minimize the number of reversals because it is the changes in momentum incidental to axial reversal that causes the greatest wear in axially reciprocating rollers. Because the square thread pitch had to be quite large in the patented embodiment, small rollers such as two inch diameter rollers using the totally self-contained oscillating mechanism described in the patent were not acceptable. Thus, presses having sizes requiring small diameter rollers were deprived of the benefits of the new oscillating rollers. A roller having a 1/4 inch helix pitch and a four inch diameter and a peripheral or lineal speed of 600 feet per minute will be rotating at 1,145 rpm. If the strikers in the helixes are located so that the roller shifts axially two inches and then returns to its original position, it will reverse 128 times per minute or over two times per second. A two inch diameter roller having a 1/4 inch thread pitch and having a peripheral or lineal speed of 600 feet per minute would rotate at 4,584 rpm and would reverse 256 times per minute. Experience showed that large rollers, such as four inch diameter rollers, having 1/4 inch thread pitch produced acceptable vibration when rotating at speeds up to 1,500 lineal feet per minute or about 1,435 rpm.
A two inch roller, having a peripheral speed of 1,500 feet per minute would be rotating at about 2,865 rpm and if the thread pitch were 1/4 inch and the axial travel of the roller were two inches to the right and back two inches to the left there will be 358 reversals per minute. But the problem is that many of the latest printing press models, particularly web as opposed to sheet presses, run routinely at 2,000 feet per minute or more. Some presses that can run at 2,500 feet per minute are presently in use. Future presses are expected to run at as high as 3,000 feet per minute. An oscillating roller in a press that runs at only 2,000 feet per minute and an oscillating roller having a thread pitch of 1/4 of an inch and an axial travel of two inches would reverse about 478 times per minute and such frequent reversals would tend to reduce the life of the oscillating roller.
Actually, every job run on a printing press entails different parameters. For some jobs, the amount of axial travel of the axial oscillating ink roller or rollers will be smaller or greater than for other jobs where the web feed rate is different. The amount of axial travel and the frequency of axial reversals differs between different jobs that are run on the same press. The use of different inks may dictate different axially oscillating rates or different web feed through rate. On some occasions, running the press at a very high speed results in production of heat which can thin the ink too much and produce an unsatisfactory printed product. In the patented oscillating rollers of the concealed oscillating mechanism type invented by applicant and in the prior art type that uses oscillating wobble or swash plates and gear trains to effectuate oscillation of one or more rollers when it was tolerable to speed up the web transport rate, the rate at which the rollers reciprocated axially increased proportionally and usually much more than was desirable in view of the particular printing format or artwork and the quality of the ink and other variables. In prior art presses, no means were provided for controlling the rotational rate of the axially oscillating ink roller nor the axial travel rate of the roller. The problems are becoming more aggravated as web speeds go higher and higher in every new printing press model that is manufactured.